Electrolessly metallized silver coated article

ABSTRACT

A process, article produced therefrom, and compositions are described for the reception of electroless plating onto a non-catalytic metallic surface. The process comprises contacting the non-catalytic surface with a promoter composition containing ions selected from the group of nickel, cobalt, iron and mixtures thereof and a suitable reducing agent, and thereafter contacting the substrate with an electroless plating bath comprising hypophosphite for the metallic build-up.

This is a division of application Ser. No. 803,777 filed June 6, 1977,U.S. Pat. No. 4,181,760.

BACKGROUND OF THE INVENTION

Electroless or autocatalytic plating of dielectric substrates findswide-spread utility in the preparation of such diverse articles asprinted circuits, automotive trim, mirrors, electronic devices, etc.

In the normal commercial electroless plating process, the dielectricsubstrate, which has been preferably etched by physical or chemicalmeans to improve metal adhesion, is sensitized by exposure to a solutionof stannous ions, e.g., a stannous chloride solution, and then activatedby exposure to a solution of palladium ions, e.g., a palladium chloridesolution. This activation is effected by reduction of the palladium ionsto the zero valence state by the stannous ions to form palladium metalsites or by the formation of a tin/palladium complex on the surface ofthe dielectric substrate.

Thereafter, the activated substrate is plated by exposure to anelectroless plating bath containing ions of the metal to be plated and areducing agent capable of reducing (heterogeneously) the valence stateof the plating ions present in the bulk solution to the metallic state.In conventional processes, copper is plated using an electroless platingbath comprised of copper ions and formaldehyde as a reducing agent. Inthe plating of nickel or cobalt, the reducing agent commonly used issodium hypophosphite.

More recently, processes have been developed for electroless platingwithout the necessity of using palladium or other precious metals. Forexample, in U.S. Pat. Nos. 3,772,056 and 3,772,078, and non-metallicsubstrates are coated with a solution containing non-precious metalions, i.e., ions of copper, nickel, cobalt or iron, and dried to form anadherent coating of the metal ions. Thereafter, the metal ions arereduced to the metallic state and the substrate is plated with acompatible electroless plating bath.

In U.S. Pat. No. 3,993,491 another procedure for effecting electrolessplating of non-metallic substrates without the necessity of usingpalladium or any other precious metal ions is described. In theprocesses described therein, a non-metallic substrate is contacted witha stannous and copper ions to form a stannous-cuprous complex on thesurface of the substrate. The copper ions are then reduced to theirmetallic state using a suitable reducing agent.

Still another procedure is described in U.S. Pat. No. 3,993,799. In theprocess described therein, hydrous oxide colloids of metal or metal ionsare coated on the surface of a non-metallic substrate. The substrate isthen rinsed and immersed in a solution containing a reducing agentcapable of reducing the metal ions to the metallic state (or reducingthe outer surface).

While significant cost savings are realized by coating the substratewith non-precious metal ions, as exemplified by the above disclosures,instead of with the more expensive palladium or other precious metalions, care must be exercised in the selection of the electroless platingbath used with such systems. Specifically, conventional hypophosphitebaths are not effective in the plating of nickel or cobalt onto thesurface of substrates prepared using non-precious metals (e.g., copperor silver), in a commercially suitable manner. Instead, it is necessaryin the plating of nickel and cobalt to use an electroless plating bathcontaining a stronger reducing agent such as a boron reducing agent,e.g., an amine-borane, such electroless plating baths being disclosed,for example, in U.S. Pat. No. 3,338,726, or a borohydride, as shown inU.S. Pat. Nos. 2,461,661 and 3,045,334. Such reducing agents, because oftheir relatively higher cost, diminish the commercial savings to berealized in the use of such procedures. Also, in using the precedingnon-precious metal systems, at times a lower site density is realizedthus reducing the speed and effectiveness of plating onto the preparedsubstrates.

Procedures permitting the utilization of non-precious metal activatedsubstrates while eliminating or minimizing the aforesaid disadvantagesand permitting the utilization of conventional, commercially availableelectroless plating baths would be highly desirable.

It is also well documented in the art that there are a wide variety ofmetals and alloys which are non-catalytic for initiation of conventionalelectroless plating. Typical materials which are non-catalytic includecopper, gold, silver, chromium containing stainless steels, Kovar, moly,manganese, aluminum and its alloys and other. In the prior art, exoticprocedures have been adapted to provide such non-catalytic materialscatalytic. Typical procedures well known are activated with palladium,the zincating method, impressing of a galvanic potential and others. Itis well documented that such procedures are tedious and costly and itwould be highly desirable to eliminate them by a simple treatment whichpotentially would be universal to all non-catalytic materials (metalsand alloys). Surprisingly, I found that the composition disclosed mayalso be used effectively upon these non-catalytic materials and therebyrender the surface of such material platable in conventional electrolessplating baths. In an attempt to overcome such tedious procedures, arecent effort is described in U.S. 4,002,778 which is included herein byreference. However, there is still room for improvement with regard tothe number of steps and the economy of the process used.

SUMMARY OF THE INVENTION

It is the principal object of the present invention to produce ametallized silver coated ceramic substrate prepared by the steps of

(a) contacting the substrate to be coated with a promoter compositionwhich comprises; metal ions and only a single reducing agent, saidreducing agent being capable of chemically reacting with said substrateand the metal ions within the composition and wherein said metal ionsare selected from the group of nickel, cobalt, and iron and wherein therelative concentration of the reducing agent to the metal ions is soadjusted as to permit the initial chemical interaction of the reducingagent with said substrate and then the heterogeneous reduction of someof the metal ions present in said composition, and then

(b) contacting the treated substrate with a conventional electrolessplating bath comprising hypophosphite and thereby depositing a metalliclayer.

DETAILED DESCRIPTION OF THE INVENTION

The term "priming" as used in the present description means theformation of a coating of non-precious metal ions (or metals or alloys)including silver and other non-catalytic metal onto the surface of anon-metallic (dielectric) substrate or metallic substrates. The primingstep does not per se form a part of the present invention. Priming maybe effected by one of a number of techniques including the proceduresdescribed in the above-mentioned patents. Priming may also be effectedby vapor deposition, spraying, printing, dipping, etc., or the formationof a metal in the metallic state on the substrate surface followed bypermitting or causing the surface of the metal to oxidize. For certainpurposes, priming may be on selected regions of the substrate, therebyresulting in selective plating.

Because of their particular effectiveness and commercial significance,the non-metallic substrates will normally be primed with copper ions,either cuprous or cupric, and the following description will beprimarily directed to the plating of copper and silver primed substratesas well as non-catalytic metals and alloys. It is to be understood,however, that the present invention is broadly directed to the platingof non-metallic, metallic or semiconductor substrates primed with othermetals or their ions, e.g., nickel, cobalt, iron, tin, mercury, silver,etc.

The term "developing" (or promoting) as used herein means the reductionof metal ions coated on the substrate to the metallic or zero valencestate with a chemical reducing agent capable of effecting suchreduction, or the initial interaction of the reducing agent with thenon-catalytic metal (or alloy) previously deposited onto the substrate,or which is part of the substrate with the sequential deposition ofmetal (e.g., nickel, cobalt, iron) which is derived from the bulksolution of the developing medium. Accordingly, the developing mediumwill be referred to as "developer" or "promoter". It is recognized thatthe interaction of the promoter is a surface reaction; the properties ofthe nature of the bulk substrates is of no major concern. Accordingly,the concentration of the reducing agent must be so adjusted relative tothe metal ions in the bulk solution as to insure the preferred sequenceof chemical reactions. It is also noted that the promoter compositionmay also include other additives from pH adjusters to complexing agentswhich should be obvious to one skilled in the art. It is recognized thatproper selection of complexing (or chelating) agents is importantwhereby the metal ions are complexed but still available for theheterogeneous reduction at moderate conditions (e.g., temperature and/orreducing agent concentration).

The term "non-catalytic metal" as used in the present invention refersto a wide variety of materials (metals or alloys) which are notcatalytic and hence would not initiate conventional electroless plating.Typical materials may include, however are not limited to, copper, gold,silver, chromium containing steel, stainless steels, steel Kovar, moly,aluminum and its alloys, zinc, and others.

The process and systems of the present invention are applicable to themetallic plating of a wide variety of substrates (semiconductors,dielectrics, and metallic). It is noted that some of the objectives setforth in this invention are met in a recent patent, U.S. Pat. No.3,993,801 which is included herein by reference.

In general, the process of the present invention comprises the followingsteps:

(A) Priming of the substrate, which has preferably first been etched,with a metal alloy or metal ions, preferably copper and silver ornon-catalytic metals,

(B) Immersing said substrate in a promoter composition containing metalions selected from nickel, cobalt, iron and copper ions and mixturesthereof, and a reducing agent capable of reducing or interacting firstwith the metal or metal ions on the substrate and then heterogeneouslyreducing the ions in the promoter bulk solution to the metal state, and

(C) Electrolessly plating said substrate by immersing said substrate inan electroless plating bath containing ions of the metal to be platedand a reducing agent capable of reducing heterogeneously the valencestate of the ions in the electroless plating bath to the metallic state.

It is recognized that when dealing with bulk metals or alloys, thepriming step does not exist per se. More specifically, in the plating ofnickel or cobalt onto a substrate primed with silver or othernon-catalytic metals, the process comprises the following steps:

(A) Immersing the primed substrate in a promoter solution containingions of nickel or cobalt and a reducing agent capable of reducing thesilver ion or interacting with metallic silver on said substrate andsaid nickel or cobalt ions to the metallic state; and

(B) Immersing the developed substrate into an electroless plating bothcontaining nickel or cobalt ions and a reducing agent capable ofreducing said nickel or cobalt ions to their metallic state, e.g., ahypophosphite.

As noted previously, it is another specific object of the presentinvention to provide improved electroless copper plating onto copperprimed substrates. This objective may be accomplished by developing thecopper primed substrate with the developer solution described above forthe plating of nickel or cobalt, or a similar bath containing copperions, followed by immersion of the developed substrate in a conventionalelectroless copper-formaldehyde bath.

It is believed that improved copper plating is achieved using the abovedeveloper solutions through intensification of the sites on thesubstrate due to plating of copper, nickel or cobalt from the developersolution onto the substrate. Such intensification appears to be effectedby the deposition onto the substrate of a thin layer, i.e., less than1000 A in thickness on the surface may be sufficient; however, forcertain applications one may extend to greater thickness.

Suitable reducing agents used in the promoter solutions of the presentinvention may be any chemical reducing agent capable of reducing theions on the substrate (or chemically interacting with the metal presenton the surface) and also in the developer solution to the metallicstate. In the chemical interaction, generally hydrogen gas is formedthrough the presence of a reactive intermediate adsorbed on the surface.While I do not wish to be bound by theory, the adsorbed intermediate maybe chemisorbed nascent hydrogen or a hydride ion, both of which areshort lived. Exemplary of such reducing agents are amine-boranes,borohydrides, hydrazine and its derivatives, N-alkyl-borazones,N-alkyl-borazoles, borazenes, borazines, and mixtures thereof.Particular reducing agents include dimethylamine borane, diethyleneborane, and the alkali metal and alkaline earth metal borohydrides, suchas potassium and sodium borohydrides.

The following are a few publications describing the use of miscellaneousreducing agents (e.g., hydrazine) which have been reported capable ofboth copper and nickel plating.

P. Fintschenko et al, Metal Finishing, January (1970).

D. J. Levy, Proc. Electroplaters Soc., 50, p. 29 (1963).

D. J. Levy, Electrochem. Tech., 1, No. 1-2, p. 38 (1963).

J. W. Dini et al, Plating, 54, p. 385 (1967).

While not wanting to be held to any particular theory, it is believedthat treatment of the primed substrate with the promoter solutionresults in the reducing agent present in the developer solution firstreducing the copper ions (or inter-acting with the metallic copper)present on the surface of the substrate to their metallic state (or anactivated state), the reduction reaction being indicated by theformation of a brown color on the substrate. Thereafter, additionalreducing agent in the developer solution heterogeneously reduces thevalence state of the ions in the bulk developer solution to the metallicstate causing plating of the metal onto the substrate. In the case ofnickel, this latter step is indicated by the formation of a greyishcolor on the substrate. Accordingly, sufficient reducing agent should bepresent in the developer solution to insure the sequential reaction withthe primed surface and thereafter heterogeneously reduce the ions in thedeveloper solution. Thus for increased probability, a molar ratio ofreducing agent to metal ions in the developer solution should exceed1:1, and preferably should be at least 2:1. Ratios greater than about15:1, while workable, are of little practical value and serve toincrease the cost of the process. The molar concentration of thereducing agent will normally be within the range of from about 0.015 Mto about 0.2 M; and the molar concentration of the metal ions willnormally be within the range of from about 0.003 M to about 0.1 M.Surprisingly, it is noted that the concentration of reducing agents usedin electroless plating baths (boron type), as referred in the prior art,all normally range from about 0.015 to about 0.2 m/l, while the metalion concentration will range from about 0.02 to about 0.5 m/l. The molarratio of reducing agent to metal ions, thus, is less than 1:1, andnormally between 0.75 and 0.4. Such baths are taught, for example, inU.S. Pat. No. 3,338,726 as typical compositional make-up which areeffective for electroless metal build-up.

Conventional electroless plating baths suitably used in plating inaccordance with the present invention are comprised of ions of the metalto be plated, a complexing agent, and a reducing agent. In nickel orcobalt baths, the reducing agent commonly employed is a hypophosphitereducing agent, such as sodium hypophophite; in copper baths, thereducing agent commonly employed is formaldehyde or other electroless(chemical) plating baths which are not compatible with the substrates tobe plated.

In the preparation of such baths, the metal ions are suitably derivedfrom salts of the metal, e.g., the chloride or sulfate salts. Suitablecomplexing agents are well known in the art and include ethylenediamenetetraacetate, citrate and ammonia.

The following examples are presented as illustrative of the presentinvention and not in limitation thereof. In the examples where nickelions are used in the developer or plating solutions, it will be apparentto one skilled in the art that cobalt ions, because of their similarproperties, may be substituted.

EXAMPLE I

In this example as well as the following examples, the followingprocedure was employed:

1. Immerse ABS substrates, previously etched with a CrO₃ /H₂ SO₄solution, into the described primer solution for several minutes;

2. Rinse;

3. Immerse primed substrate into the described developer solution;

4. Rinse (optional); and

5. Immerse developed substrate into described electroless plating bath.

In the present example, a primer solution having the followingcomposition was used at room temperature:

    ______________________________________                                        SnCl.sub.2.2H.sub.2 O 81    g/l                                               CuCl                   6    g/l                                               HCl (conc.)           45    cc/l                                              Phenol                40    g/l                                               ______________________________________                                    

Following immersion in the above primer solution, the primed substrateswere rinsed and immersed in the following developer (promoter) solution:

    ______________________________________                                        Dimethylamine borane 3         g/l                                            (DMAB)                                                                        NiSO.sub.4.6H.sub.2 O                                                                              2.5       g/l                                            Citric acid.H.sub.2 O                                                                              3.6       g/l                                            NH.sub.4 OH to pH    8.8                                                      Temperature          36° C.                                            ______________________________________                                    

It was observed that within 2-3 minutes the surface becomes brown incolor, and within 3-5 minutes a complete intensification took place asshown by a grey color.

Nickel plating was achieved by immersion of the developed substrate inthe following electroless nickel-hypophosphite bath:

    ______________________________________                                        Bath 1      NiSO.sub.4.6H.sub.2 O                                                                         12.5    g/l                                                   Citric acid.H.sub.2 O                                                                         18      g/l                                                   NH.sub.4 OH to pH                                                                             8.9                                                           NaH.sub.2 PO.sub.2.H.sub.2 O                                                                  18      g/l                                                 Temperature    C.                                                   ______________________________________                                    

As aforementioned, improved copper plating can also be achieved usingthe present improved developer solutions due to intensified sitedevelopment. Thus, uniform plating of copper was achieved by immersionof a substrate developed in the foregoing manner into a conventionalelectroless copper-formaldehyde bath having the following composition:

    ______________________________________                                        Bath 2    CuSO.sub.4.5H.sub.2 O                                                                             10     g/l                                                KNaC.sub.3 H.sub.4 O.sub.6.4H.sub.2 O                                                             16     g/l                                                (potassium sodium tartrate)                                                   NaOH                16     g/l                                                H.sub.2 CO (37%)     8     g/l                                      ______________________________________                                    

EXAMPLE II

Electroless plating of nickel and copper was obtained using theprocedure, primer solution and electroless plating baths of Example Iwith the following developer solution:

    ______________________________________                                        DMAB                  3       g/l                                             CoSO.sub.4.7H.sub.2 O 1.25    g/l                                             Sodium citrate.2H.sub.2 O                                                                           2.5     g/l                                             NH.sub.4 OH to pH     8.8                                                     Temperature           36° C.                                           ______________________________________                                    

EXAMPLE III

Electroless plating of nickel and copper was obtained using theprocedure, primer solution and electroless plating baths of Example Iwith the following developer solution:

    ______________________________________                                        DMAB                  3       g/l                                             CoSO.sub.4.7H.sub.2 O 1.25    g/l                                             Sodium citrate.2H.sub.2 O                                                                           2.5     g/l                                             CuSO.sub.4.5H.sub.2 O 0.072   g/l                                             NH.sub.4 OH to pH     8.8                                                     Temperature           36° C.                                           ______________________________________                                    

It should be noted that this composition is more reactive in comparisonto the composition of Example II and thus lowering of the reactivity isrecommended. Moreover, it should be obvious to those skilled in the artof plating that the catalytic surface resulting at the conclusion of thedevelopment stage consists of both cobalt and copper.

EXAMPLE IV

Electroless plating of nickel and copper was obtained using theprocedure, primer solution and electroless plating baths of Example Iwith the following developer solution:

    ______________________________________                                        DMAB                3       g/l                                               Nickel sulfamate    0.8     g/l (Ni)                                          NH.sub.4 OH to pH   8                                                         Temperature         38° C.                                             ______________________________________                                    

Good intensified development took place within 5 minutes. AdditionalTergitol (TMN) surfactant seemed to improve the overall uniformity.

EXAMPLE V

In this example, priming of the ABS substrate was achieved using as theprimed solution a hydrous oxide colloid of copper prepared by adding 400ml of 0.025 molar NH₄ OH dropwise with stirring to 1600 ml of 0.0125molar copper acetate.

ABS substrates primed with the above colloidal solution were developedusing the following developer solution:

    ______________________________________                                        DMAB                 4     g/l                                                Nickel sulfamate     1.6   g/l (Ni)                                           NaOH to OH           6.2                                                      Temperature          44° C.                                            ______________________________________                                    

Using the electroless nickel bath of Example I, a complete intensifieddeveloped surface was obtained within 5 minutes of immersion, and goodinitiation in the electroless bath was noted. It should be noted thatusing a modified developer formulation similar to Example No. 1 waspoor, probably due to the presence of ammonia. Based upon this exampleand procedure, it should be obvious that hydrous oxide colloids ofcobalt and nickel may be used as well as combinations thereof.

EXAMPLE VI

    ______________________________________                                        DMAB                1.5      g/l                                              NiSO.sub.4.6H.sub.2 O                                                                             1.25     g/l                                              Citric acid.H.sub.2 O                                                                             1.8      g/l                                              NH.sub.4 OH to pH   7.8                                                       Temperature         37° C.                                             ______________________________________                                    

When substituting the above solution after a partial degassing for thedeveloper solution of Example V, it was noted that development tookplace within 2 minutes while complete intensification took place inabout 8 minutes. In this example, no agitation or surfactant wasincluded. The intensified developed substrate was directly immersed intothe electroless nickel bath of Example I with good immediate initiationnoted. Dilution (×2) of the above modified developer formulation underthe same conditions did result in intensified development, however witha lower speed.

EXAMPLE VII

Electroless plating of nickel was obtained using the procedure, primersolution and electroless nickel plating bath of Example I with thefollowing developer solution:

    ______________________________________                                        NiCl.sub.2.6H.sub.2 O 3       g/l                                             Ethylene diamine      5       g/l                                             Potassium borohydride 1       g/l                                             pH                    9.9                                                     Temperature           38° C.                                           ______________________________________                                    

Standard development was noted within two to three minutes of immersion,while complete intensification was observed only after about twelveminutes of immersion. The latter could be foreshortened by furtheradjusting developer reactivity and probably by lowering or eliminatingthe ethylenediamene concentration. Following the intensified developmentgood initiation in the electroless bath No. 1 took place. To overcomesome of the stability problems associated with borohydrides, the use ofsalts of cyano-borohydrides is recommended. The latter show goodstability over a wide pH range.

EXAMPLE VIII

As stated previously, one of the novel features of this invention is thefact that development and intensification take place in the same mediumin a preferred sequence of events. This feature is accomplished to alarge extent by the relative concentration make-up of the developersolution. To better illustrate this point the following results areprovided.

    ______________________________________                                                     Observed timing (minutes) to:                                         DMAB (a)/     Development                                                                              Intensified                                          NiSO.sub.4.6H.sub.2 O(g/g)                                                                  (brown color                                                                             Development                                     No.  in modified solution                                                                        formation) (grey color formation)                          ______________________________________                                        1    3/12.5        (b)        none after 7 min.                               2    3/6.25        (b)        40% only after 7 min.                           3    3/3.12        21/2       4                                               4    3/1.6         21/2       4                                               ______________________________________                                          (a) All developer solutions were operated at 39° C. and were also     composed of citric acid H.sub. 2 O at ×1.44 the weight of nickel        sulfate.hexahydrate. Ammonium hydroxide was used to maintain a pH of 8.7.      (b) The observation of a brown color was nonreproducible and in cases in     which a brown color was formed, the intensified development was sluggish.

EXAMPLE IX

A ceramic substrate coated with silver was used; the silver was derivedfrom a printable ink composition. After activation in a 20% hydrochloricacid, the substrate was effectively treated in the developer of exampleI and then in electroless nickel-phosphorus formulation. It isrecognized that in using surface primed with silver, it is advantageousto include within the developer composition a silver complexing agent.Such inclusion will extend the life of the developer composition, hencemaking the process more economical.

I have also found that due to the non-noble (precious) characteristic ofsilver it may be required to treat the silver surface prior to thedeveloper composition as to dissolve any oxide layer.

EXAMPLE X

Cleaned non-catalytic materials such as moly, chromium, copper, chromiumcontaining steel, and an alloy of aluminum (7075 T6, 6061 T6), werecontacted for 1 to 2 minutes in a composition of Example I attemperature range 60° to 70° C. After this treatment, the materials wereimmersed in a conventional electroless nickel using hypophosphite as thereducing agent. It was noted that instantaneous plating was started. Theplated parts surprisingly have also shown good adhesion before and afterheat treatment.

What I claim is:
 1. A metallized article, said article being a silvercoated ceramic substrate, produced by the steps comprising,(a)contacting said silver coated ceramic substrate with a compositionthereby rendering said substrate receptive to conventional electrolessplating and wherein said composition comprises metal ions and only asingle reducing agent and wherein said reducing agent is capable ofchemically reacting with said silver coated ceramic substrate and themetal ions within said composition and wherein the metal ions areselected from the group consisting of nickel, cobalt, and iron andmixtures thereof, and wherein the relative molar concentration of thereducing agent to the metal ions is so adjusted as to permit the initialchemical interaction of the reducing agent with said silver coatedceramic substrate and then the heterogeneous reduction of some of themetal ions present in said composition and then, (b) contacting thetreated silver coated ceramic substrate with a conventional electrolessplating bath thereby depositing a metallic layer and wherein said bathcomprises hypophosphite.
 2. The article according to claim 1 whereinsaid silver is derived from a printable ink composition comprisingsilver.
 3. The article according to claim 2 wherein said reducing agentis selected from the group consisting of amineboranes, borohydrides, andhydrazine.
 4. The article according to claim 1 wherein said relativemolar concentration of reducing agent to metal ions is at least 1:1. 5.The article according to claim 1 wherein said composition is used at analkaline pH.
 6. The article according to claim 1 wherein saidelectroless plating bath comprises nickel ions.
 7. The article accordingto claim 1 wherein said composition is used above room temperature.